Although it may not appear to be so, Parallax Software DID produce D3 - or rather, one half of it. Parallax Software, when originally formed in 1993, had people in two locations (Ann Arbor, Michigan and Champaign, Illinois).
For Descent, work was done in Champaign while the company searched for a mutual central location. After failing to find one, it was decided to reorganize Parallax into two companies, Outrage (Ann Arbor) and Volition Inc. (Champaign). Both companies still maintain strong links to each other.